The main aims of the proposed work are to identify and characterize the external and internal factors that mediate seasonal cycles in reproduction and energy balance and to elaborate the neural substrates that generate circadian rhythms in behavior and physiology of mammals. The model species to be studied are the meadow vole and the Syrian hamster. Specific projects include 1) determining the role of testicular hormones in mediating the decline in body weight and food intake that occurs during fall and winter, 2) the contribution of gonadal hormones to individual differences in food intake and body weight, 3) the role of photoperiod in inducing refractoriness to the weight-promoting effects of androgens, 4) the influence of the pineal gland in mediating the effects of photoperiod on energy balance, 5) specification of the neural pathways by which photoperiod influences food intake and body mass, 6) the role of photoperiod in counteracting the obesities produce by ovariectomy or by destruction of the ventromedial nuclei of the hypothalamus, 7) specification of the degree of redundancy in the mediation of circadian rhythms by the suprachiasmatic nuclei. Parameters to be measured include body weight, food intake, locomotor activity, testosterone levels in plasma and several other indices of reproductive status or energy balance. The proposed research will develop concepts and data relevant to understanding and manipulation of human circadian rhythms as well as for the control of food intake and body mass through non-invasive means.